A randomized clinical trial was conducted to examine the effect of antenatal vitamin A supplementation on vertical transmission of HIV, birth weight, and other health outcomes in Malawi. Preliminary analysis shows that cumulative vertical transmission rates were 27.3 percent in the vitamin A and 32.0 percent in the placebo group (p=0.25) by 12 months of age. At this time, greater than 99 percent of women were still breastfeeding their infants. The incidence of low birth weight (less that 2500 gms) was 14.0 percent and 21.1 percent in the vitamin A and placebo groups, respectively (p less than 0.03). Vitamin A significantly reduced low birth weight in a multivariate analysis controlling for maternal age, body mass index, and CD4+ count. During this study, preliminary studies were conducted to examine the relationship between mastitis, breastmilk HIV load, and vertical transmission. Mastitis is an inflammatory condition in the breast characterized by an opening of paracellular pathways in milk secretory glands. Serum-derived components such as sodium and inflammatory cells such as lymphocytes (presumably including HIV-infected lymphocytes and infectious virions) enter the breast milk. Breastmilk sodium levels are a sensitive indicator of mastitis. At 6 weeks post-delivery, 16.4 percent of women had an elevated breastmilk sodium level consistent with mastitis. Vertical transmission to 6 weeks of age was 45.4 percent among women with elevated breastmilk sodium versus 23.6 percent among women with normal breastmilk sodium (p less than 0.001). Elevated breastmilk sodium was associated with higher vertical transmission (O.R. 2.38, 95 percent C.I. 1.29-4.39, p less than 0.005), adjusting for maternal plasma HIV load. In a nested substudy, women with (cases) and without (control) elevated breastmilk sodium were matched by plasma HIV load and CD4+ lymphocyte count. Cases has a median breastmilk HIV load of 1765 copies/mL compared with undetectable load in controls (p less than 0.0001). Higher breastmilk HIV load was associated with higher vertical transmission (p less than 0.007). Mastitis appears to be an important risk factor for vertical transmission of HIV in breastfeeding populations. In the competing continuation, follow-up of mothers and infants in the clinical trial will be completed until the end of breastfeeding. The relationship between mastitis and breastmilk HIV load and the epidemiology and microbiology of mastitis will be examined in a new cohort of HIV-infected lactating women.